Extremity imaging apparatus for cone beam computed tomography

ABSTRACT

A system for cone beam computed tomography of an extremity has an x-ray source and a digital radiographic detector both revolved about a central imaging axis so that the detector moves at least partially around a first extremity of a patient and, if the extremity is the leg of the patient, the detector moves between the legs of the patient. The detector path has a radial distance less than that of the source path. A housing that encloses the source and detector includes a pivoting door to allow sideways access for the patient&#39;s leg to be placed at the central imaging axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 15/424,935, filed Feb. 6, 2017, in the name of Yorkston et al.,entitled EXTREMITY IMAGING APPARATUS FOR CONE BEAM COMPUTED TOMOGRAPHY,which is a continuation of U.S. patent application Ser. No. 15/012,995,filed Feb. 2, 2016, in the name of Yorkston et al., entitled EXTREMITYIMAGING APPARATUS FOR CONE BEAM COMPUTED TOMOGRAPHY, which is acontinuation of U.S. Pat. No. 9,277,899, filed Mar. 5, 2015, in the nameof Yorkston et al., entitled EXTREMITY IMAGING APPARATUS FOR CONE BEAMCOMPUTED TOMOGRAPHY, which is a continuation of U.S. Pat. No. 8,998,486,filed May 1, 2014, in the name of Yorkston et al., entitled EXTREMITYIMAGING APPARATUS FOR CONE BEAM COMPUTED TOMOGRAPHY, which is acontinuation of U.S. Pat. No. 8,746,972, filed Nov. 20, 2012, in thename of Yorkston et al., entitled EXTREMITY IMAGING APPARATUS FOR CONEBEAM COMPUTED TOMOGRAPHY, which is a continuation of U.S. Pat. No.8,348,506, filed Apr. 30, 2010, in the name of Yorkston et al., entitledEXTREMITY IMAGING APPARATUS FOR CONE BEAM COMPUTED TOMOGRAPHY, whichclaims the benefit of U.S. Ser. No. 61/175,091 provisionally filed onMay 4, 2009, in the names of Yorkston et al., entitled Cone BeamComputed Tomography (CBCT) For Extremity Imaging, which is herebyincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to diagnostic imaging and in particularto cone beam imaging systems used for obtaining volume images ofextremities.

BACKGROUND OF THE INVENTION

3-D volume imaging has proved to be a valuable diagnostic tool thatoffers significant advantages over earlier 2-D radiographic imagingtechniques for evaluating the condition of internal structures andorgans. 3-D imaging of a patient or other subject has been made possibleby a number of advancements, including the development of high-speedimaging detectors, such as digital radiography (DR) detectors thatenable multiple images to be taken in rapid succession.

Cone beam (CB) computed tomography (CT) (CBCT) or cone beam CTtechnology offers considerable promise as one type of diagnostic toolfor providing 3-D volume images. Cone beam CT systems capture volumetricdata sets by using a high frame rate digital radiography (DR) detectorand an x-ray source, typically affixed to a gantry that rotates aboutthe object to be imaged, directing, from various points along its orbitaround the subject, a divergent cone beam of x-rays toward the subject.The CBCT system captures projections throughout the rotation, forexample, one 2-D projection image at every degree of rotation. Theprojections are then reconstructed into a 3D volume image using varioustechniques. Among the most common methods for reconstructing the 3-Dvolume image are filtered back projection approaches.

Although 3-D images of diagnostic quality can be generated using CBCTsystems and technology, a number of technical challenges remain. In somecases, for example, there can be a limited range of angular rotation ofthe x-ray source and detector with respect to the subject. CBCT Imagingof legs, arms, and other extremities can be hampered by physicalobstruction from a paired extremity. This is an obstacle that isencountered in obtaining CBCT image projections for the human leg orknee, for example. Not all imaging positions around the knee areaccessible; the patient's own anatomy prevents the radiation source andimage detector from being positioned over a portion of the scancircumference.

To illustrate the issues faced in CBCT imaging of the knee, the top viewof FIG. 1 shows the circular scan paths for a radiation source 22 anddetector 24 when imaging the right knee R of a patient as a subject 20.Various positions of radiation source 22 and detector are shown indashed line form. Source 22, placed at some distance from the knee, canbe positioned at different points over an arc of about 200 degrees; withany larger arc, left knee L blocks the way. Detector 24, smaller thansource 22 and typically placed very near subject 20, can be positionedbetween the patient's right and left knees and is thus capable ofpositioning over the full circular orbit.

A full 360 degree orbit of the source and detector is not needed forconventional CBCT imaging; instead, sufficient information for imagereconstruction can be obtained with an orbital scan range that justexceeds 180 degrees by the angle of the cone beam itself, for example.However, in some cases it can be difficult to obtain much more thanabout 180 degree revolution for imaging the knee or other joints andother applications. Moreover, there can be diagnostic situations inwhich obtaining projection images over a certain range of angles hasadvantages, but patient anatomy blocks the source, detector, or bothfrom imaging over that range.

For imaging the leg, one way around this problem is to arrange thepatient in a pose such that the subject leg is extended into a CBCTscanning apparatus and the paired leg is supported in some other way orbent with respect to the subject leg, such as at a right angle. This isthe approach used, for example, in the CT scanner device taught in U.S.Pat. No. 7,394,888 entitled “CT Scanner for Lower Extremities” toSukovic et al. In the methods of the Sukovic et al. '888 disclosure, theother leg must either be lifted out of place or spread at a distance, oris relaxed while the subject leg is lifted out of place and extendedinto the scanner equipment. This arrangement can be particularlydisadvantageous for a number of reasons. It can be helpful, for example,to examine the condition of a knee or ankle joint under the normalweight load exerted on that joint by the patient. But, in requiring thepatient to assume a position that is not usually encountered in typicalmovement, the Sukovic et al. '888 apparatus may obtain an image whenthere is excessive strain, or insufficient strain, or poorly directedstrain, on the joint.

Another issue with conventional approaches relates to imaging of aload-bearing extremity such as the human leg. Because of the inabilityto image the leg under a normal load, as the patient is in a standingposition, various artificial ways of mimicking load conditions have beenattempted. Such approaches have used various types of braces,compression devices, and supports. As one example intended to remedy theshortcomings of conventional imaging techniques, the Sukovic et al. '888disclosure teaches simulating the normal loading of the leg by elevatingthe leg to a non-standing position, then applying an external forceagainst the leg. However, it can be readily appreciated that while thistype of simulation allows some approximation of load-bearing limbresponse, it can be inaccurate. The knee or ankle joint, under someartificially applied load and at an angle not taken when standing, maynot behave exactly as it does when bearing the patient's weight in astanding position.

Another difficulty with the Sukovic et al. '888 apparatus and with otherdevices designed to address knee and lower leg imaging relates to poorimage quality. For image quality, the CBCT sequence requires that thedetector be up close to the subject and the source of the cone beamradiation be at a sufficient distance from the subject. This providesthe best image and reduces image truncation and consequent lost data.Positioning the subject midway between the detector and the source, asSukovic et al. '888 apparatus and with other devices require, not onlynoticeably compromises image quality, but also places the patient toonear the radiation source, so that radiation levels are considerablyhigher. One example of this strategy is shown in German patentpublication DE 10146915. With the C-shaped gantry arrangement shown,centering the subject at the center of rotation of source and detectorwould apply considerably higher radiation amounts with each projectionand severely compromise image quality. Any other positioning of thesubject, such as closer to the detector, might reduce radiation levelsover some part of the image capture sequence, but would result in undulycomplex image reconstruction problems, since this would actually varythe distances between radiation source and subject and between subjectand detector with each projection image obtained. Attempted imaging ofthe knee with such a system would require the patient to be supported insome way, balancing on the leg being imaged. It can be appreciated thatthis requirement is unreasonable or impossible for many situations inwhich an injured knee is being imaged. Thus, the C-shaped gantry shownwould not be suitable for imaging only one knee of the patient.

Imaging of the foot and ankle presents additional obstacles for CBCTprojection image capture. Approaches such as that given in the Sukovicet al. '888 disclosure, centering the foot between source and detector,suffer from the same problems of poorly positioned exposure andnoticeably compromised image quality.

In summary, for extremity imaging, particularly for imaging the lowerpaired extremities, a number of improvements are needed, including thefollowing: (i) improved placement of the radiation source and detectorto provide acceptable radiation levels and image quality throughout thescanning sequence; (ii) system flexibility for imaging at differentheights with respect to the rotational axis of the source and detector,including the flexibility to allow imaging with the patient standing orseated comfortably, such as with a foot in an elevated position, forexample; (iii) improved patient accessibility, so that the patient doesnot need to contort, twist, or unduly stress limbs or joints that mayhave been injured in order to provide images of those body parts; (iv)improved ergonomics for obtaining the CBCT image, allowing the patientto stand with normal posture, for example. This would also allowload-bearing extremities, such as legs, knees, and ankles, to be imagedunder the normal load exerted by the patient's weight, rather than undersimulated loading conditions as taught in the Sukovic et al. '888disclosure and elsewhere.

Thus, it can be seen that although a number of solutions have beenproposed to address the problem of CBCT extremity imaging, conventionalsolutions fall short of what is needed for both usability andperformance.

SUMMARY OF THE INVENTION

It is an object of the present invention to advance the art ofdiagnostic imaging of extremity body parts, particularly jointed orload-bearing, paired extremities such as knees, legs, ankles, fingers,hands, wrists, elbows, arms, and shoulders.

It is a feature of the present invention that it provides an apparatuswith different radii for orbital paths of sensor and radiation sourcecomponents.

It is an advantage of the present invention that it allows imaging ofload-bearing extremities for a patient who is standing.

From one aspect, the present invention provides apparatus for cone beamcomputed tomography of an extremity of a patient, the apparatuscomprising: a digital radiation detector; a first device to move thedetector along at least a portion of a circular detector path, theportion of the detector path extending so that the detector moves bothat least partially around a first extremity of the patient and betweenthe first extremity and a second, adjacent extremity of the patient, thedetector path having a radius R₁ that is sufficiently long to allow thefirst extremity of the patient to be positioned approximately at acenter of the detector path; a radiation source; a second device to movethe source along at least a portion of a concentric circular source pathhaving a radius R₂ greater than radius R₁, radius R₂ being sufficientlylong to allow adequate radiation exposure of the first extremity for animage capture by the detector; and a first circumferential gap in thesource path to allow the second extremity to be positioned in the firstcircumferential gap during the image capture.

According to another aspect, the present invention provides an apparatusfor cone beam computed tomography of a portion of a subject leg of apatient who is standing on the subject leg and its paired leg, theapparatus comprising: a digital radiation detector; a detector transportthat defines a detector path for movement of the digital radiationdetector along a first circular arc, wherein the first circular arc hasa radius R1 with respect to a central point within the subject leg andwherein the first circular arc extends through the space between thesubject leg and its paired leg; a radiation source; a radiation sourcetransport that defines a radiation source path for movement of theradiation source along a second circular arc of a second radius R2,larger than radius R1, with respect to the central point in the subjectleg, wherein the second circular arc lies outside the space between thesubject leg and its paired leg; and a circumferential gap in theradiation source path for placement of the subject leg.

These objects are given only by way of illustrative example, and suchobjects may be exemplary of one or more embodiments of the invention.Other desirable objectives and advantages inherently achieved by thedisclosed invention may occur or become apparent to those skilled in theart. The invention is defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of the embodiments of the invention, as illustrated in theaccompanying drawings. The elements of the drawings are not necessarilyto scale relative to each other.

FIG. 1 is a schematic view showing the geometry and limitations of CBCTscanning for portions of the lower leg.

FIG. 2 shows a top and perspective view of the scanning pattern for animaging apparatus according to an embodiment of the present invention.

FIG. 3 is a perspective view showing patient access to an imagingapparatus according to an embodiment of the present invention.

FIG. 4 is a perspective view showing the patient in a scanning position.

FIG. 5 is a series of top schematic views showing the sequence forpatient access and system preparation for CBCT imaging.

FIG. 6 is a series of top schematic views showing the sequence forobtaining CBCT projections at a number of angular positions.

FIG. 7 is a perspective view showing optional height adjustment.

FIGS. 8A and 8B are perspective views that show extremity imaging for anextended leg in an alternate configuration.

FIG. 9 is a perspective view that shows a configuration of the imagingapparatus for upper extremity imaging.

FIG. 10 is a perspective view that shows imaging with the detectortransport fully encircling the lower extremity.

FIG. 11 is a perspective view that shows imaging with the detectortransport fully encircling the upper extremity.

FIG. 12A shows perspective views of imaging apparatus with and withoutcovers.

FIG. 12B is a perspective view of an imaging apparatus using a turntablefor source and detector transport.

FIG. 13 is a top view of the transport arrangement shown in FIG. 12B.

FIG. 14A shows a top view of the imaging apparatus with the hoodpartially transparent.

FIG. 14B shows internal components in start and stop scan positions.

FIG. 15 shows top views of the turntable transport arrangement forinitial positioning of the extremity of the patient and beginning ofscan.

FIG. 16 shows a top view during the scan sequence.

FIG. 17 shows perspective views of an embodiment for extremity imagingat a horizontal position.

FIG. 18 is a top view that compares angular considerations for foot andknee imaging.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description of the preferred embodiments ofthe invention, reference being made to the drawings in which the samereference numerals identify the same elements of structure in each ofthe several figures.

In the context of the present disclosure, the term “extremity” has itsmeaning as conventionally understood in diagnostic imaging parlance,referring to knees, legs, ankles, fingers, hands, wrists, elbows, arms,and shoulders and any other anatomical extremity. The term “subject” isused to describe the extremity of the patient that is imaged, such asthe “subject leg”, for example. The term “paired extremity” is used ingeneral to refer to any anatomical extremity wherein normally two ormore are present on the same patient. In the context of the presentinvention, the paired extremity is not imaged; only the subjectextremity is imaged.

To describe the present invention in detail, the examples given hereinfor embodiments of the present invention focus on imaging of theload-bearing lower extremities of the human anatomy, such as the leg,the knee, the ankle, and the foot, for example. However, these examplesare considered to be illustrative and non-limiting.

In the context of the present disclosure, the term “arc” or,alternately, “circular arc”, has its conventional meaning as being aportion of a circle of less than 360 degrees or, considered alternately,of less than 2π radians for a given radius.

Embodiments of the present invention address the difficulties of lowerextremity imaging by providing an imaging apparatus that defines orbitalsource and detector paths, concentric about a center point, whereincomponents that provide the source and detector paths are configured toallow patient access prior to and following imaging and configured toallow the patient to stand with normal posture during the CBCT imagecapture series. In embodiments of the present invention, this capabilityis effected by using a detector transport device that has acircumferential access opening allowing positioning of the extremity,wherein the detector transport device is revolved about the positionedextremity once it is in place, enclosing the extremity as it is revolvedthrough at least a portion of the scan.

It is instructive to consider dimensional attributes of the human framethat can be considerations for design of CBCT equipment for scanningextremities. For example, an adult human patient of average height in acomfortable standing position has left and right knees generallyanywhere from about 10 to about 35 cm apart. For an adult of averageheight, exceeding about 35-40 cm (14-15.7 inches) between the kneesbecomes increasing less comfortable and out of the range of normalstanding posture. It is instructive to note that this constraint makesit impractical to use gantry solutions such as that shown in DE10146915, described earlier, for knee imaging. Either the source or thedetector must be able to pass between the legs of a standing patient forknee CBCT imaging, a capability not available with gantry or otherconventional solutions.

The perspective and top views of FIG. 2 show how the scanning pattern isprovided using various embodiments of a CBCT imaging apparatus 10according to the present invention. A detector path 28 of a suitableradius R1 from a central axis A is provided by a first device, adetector transport 34. A source path 26 of a second, larger radius R2 isprovided by a second device, a source transport 32. The extremity,subject 20, is substantially centered along central axis A so thatcentral axis A can be considered as a line through points in subject 20.The limiting geometry for image capture is due to the arc of sourcetransport 32, blocked by patient anatomy, such as by a paired limb, totypically about 200 degrees, as noted previously. This defines a partialcircular sector, bounded by this arc and radii at start and end-of-scan.

Detector transport 34, while capable of a fully circular orbit becauseit can be moved between the standing patient's legs, follows thenecessary complementary arc to that of source transport 32. Patientaccess before scanning is eased by providing a circumferential gap 38 indetector transport 34. With detector transport 34 in the open positionshown in FIG. 3, the patient can freely move in and out of position forimaging. When the patient is properly in position, detector transport 34is revolved about axis A, substantially 180 degrees. This orbitalmovement confines the extremity more narrowly and places detector 24,not visible in FIGS. 2-4 due to the detector transport 34 housing, inposition near subject 20 for obtaining the first projection image insequence.

Circumferential gap 38 not only allows access for positioning of thesubject leg or other extremity, but also allows sufficient space for thepatient to stand in normal posture during imaging, placing the subjectleg for imaging in the central position of axis A (FIG. 2) and thenon-imaged paired leg within the space defined by circumferential gap38. Circumferential gap 38 extends approximately 180 degrees plus thefan angle, which is determined by source-detector geometry and distance.

The top views of FIG. 5 show the sequence for patient access for imagingapparatus 10. In an open access position 40, circumferential gap 38permits access of the extremity so that it can be centered in positionalong central axis A. The outline of the foot corresponding to an openaccess position 42 indicates positioning of the patient and is shown forreference. In this example, the left leg is the subject imaged; thepaired right leg would lie within or just outside circumferential gap38. Once the patient's leg or other extremity is in place, detectortransport 34, or a hooded cover or other member that defines thistransport path, can be revolved into position, closing the detectorportion of circumferential gap 38, as shown in a revolving transportposition 44. A transport in place position 46 shows detector transport34 in suitable position for executing the CBCT imaging sequence.

The top views of FIG. 6 continue the operational sequence begun in FIG.5 and show the sequence for obtaining CBCT projections at a number ofangular positions when using imaging apparatus 10. The relativepositions of radiation source 22 and detector 24, which may be concealedunder a hood, as noted earlier, are shown in FIG. 6. The source anddetector are diametrically opposite at each position during the CBCTscan and projection imaging. The sequence begins at a begin scanposition 50, with radiation source 22 and detector 24 at initialpositions to obtain an image at a first angle. Then, both radiationsource 22 and detector 24 revolve about axis A as represented in interimscan positions 52, 54, 56, and 58. Imaging terminates at an end scanposition 60. As this sequence shows, source 22 and detector 24 are indiametrically opposing positions relative to subject 20 at each imagingangle. Throughout the scanning cycle, detector 24 is within a shortdistance D1 of subject 20. Source 22 is positioned beyond a longerdistance D2 of subject 20. The positioning of source and detectorcomponents can be carried out by separate actuators, one for eachtransport path, or by a single rotatable member, as described in moredetail subsequently. It should be noted that scanning motion in theopposite direction, that is, clockwise with respect to the example shownin FIG. 6, is also possible, with the corresponding changes in initialand terminal scan positions.

Other features of imaging apparatus 10 are provided by the capability tomove both source and detector transports 32 and 34 along the axisdirection as a unit, as shown in the perspective view of FIG. 7. Avertical support 70 provides vertical transport of the imagingapparatus, so that the source and detector can be translated upwards ordownwards in the direction of the central axis in order to suit patientsof different heights and to image different portions of the leg. Theheight adjustment can be made before or after the patient's subject legto be imaged is enclosed by detector transport 34 using the setupsequence of FIG. 5.

In one embodiment, vertical support 70 also allows rotation of the CBCTimaging apparatus 10 to allow imaging of an extremity that is disposedhorizontally or is extended at some oblique angle other than vertical.FIGS. 8A and 8B show perspective views of knee imaging in a horizontalposition, with the patient seated and the leg outwardly extended. Full360 degree rotation about an axis Q is possible. It should be notedthat, with this application, similar patient accessibility applies, withdetector transport 34 revolved into position once the extremity iscentered in place. Further height adjustment is also possible, such asfor arm, elbow, or shoulder imaging, as shown in FIG. 9.

Using revolving detector transport 34 simplifies patient access andprovides sufficient imaging path for CBCT imaging, since the angularlimitation of the orbital imaging path is due to source obstruction,rather than to the detector path. Thus, for example, detector transport34 could fully encircle the limb, as shown in the examples of FIGS. 10and 11. In these embodiments, there is a circumferential gap 38 only inthe source orbit.

Referring back to the schematic diagrams of FIG. 6, radiation source 22and detector 24 each orbit the subject along an arc with radii R2 andR1, respectively. Within source transport 32, a source actuator could beused, cooperating with a separate, complementary detector actuator thatis part of detector transport 34. Thus, two independent actuatordevices, one in each transport assembly, can be separately controlledand coordinated by an external logic controller to move source 22 anddetector 24 along their respective arcs, in unison, about subject 20.

In an alternate embodiment, source and detector transport components aremechanically linked to a single revolving or rotating assembly. One sucharrangement, shown at the right in FIG. 12A and enlarged in FIG. 12B,provides source and detector transports 32 and 34 using a singlemechanical assembly, a rotating member 68, on a turntable 64 thatrevolves about central axis of rotation A and provides the needed radiifor source 22 and detector 24. As is best shown in the top view of FIG.13, detector 24 rides along the surface of the C-shaped turntable 64,orbiting the subject at radius R1. Source 22 is connected to turntable64 along an arm 66 that provides the longer radius R2. Circumferentialgap 38 extends across both source and detector paths.

It should be emphasized that the embodiments shown using rotating member68 on turntable 64 can be encased in one or more housings, therebyproviding similar appearance to imaging apparatus 10 shown in FIGS.7-11, for example. This type of arrangement has advantages for isolatingthe patient from moving components and for alleviating at least some ofthe patient anxiety that might be caused by automatically movingcomponents during imaging.

FIG. 14A shows sources and detector transports 32 and 34 and source anddetector 22 and 24 components as they are fitted within covers 80 thatprotect moving mechanical parts and help to prevent patient contact withmoving components. FIG. 14B shows the covered system with internalcomponents in begin and end scan positions 50 and 60 respectively, whenusing the scan sequence described earlier with reference to FIG. 6.

The top views of FIGS. 13, 15, and 16 show how patient access isprovided using this mechanical arrangement. Once the patient ispositioned, rotating member 68 is swung around the positioned extremity,to a start position 72, as shown at the bottom in FIG. 15. Imagingbegins at this position and continues as rotating member 68 revolvessource and detector components about axis A. For the example of FIGS. 15and 16, rotating member 68 moves in a clockwise direction.Counter-clockwise rotation would also be possible.

Rotating member 68 can also be used with an imaging configuration forupper extremities, as shown in FIG. 17. Because none of the patientanatomy blocks the transport path, a full circular orbit is permittedfor scanning with this configuration. Again, full 360 degree rotation ofthe components in the plane of rotating member 68 is possible, aboutaxis Q.

Imaging of the ankle and foot is also possible with CBCT imagingapparatus 10. However, because the foot protrudes outward into thedesired detector transport path, the allowable angular range for footimaging is more constrained than the range for leg and knee imaging. Thetop view of FIG. 18 shows, for example, that the angular range for CBCTscanning of the foot, for a standing patient, is about 50 degrees lessthan that for knee imaging, for example. A range of optional devices canalso be provided to facilitate the imaging process. For example, ahorizontal or vertical foot support can be provided for support of thepatient's foot. Optionally, the foot support can be adjustable to someoblique angle between horizontal and vertical, such as at a 33 degree or45 degree angle for example.

The invention has been described in detail with particular reference toa presently preferred embodiment, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention. The presently disclosed embodiments are thereforeconsidered in all respects to be illustrative and not restrictive. Thescope of the invention is indicated by the appended claims, and allchanges that come within the meaning and range of equivalents thereofare intended to be embraced therein.

PARTS LIST

-   10. CBCT imaging apparatus-   20. Subject-   22. Source-   24. Detector-   26. Source path-   28. Detector path-   32. Source transport-   34. Detector transport-   38. Circumferential gap-   40. Open access position-   42. Open access position-   44. Revolving transport position-   46. Transport in place position-   50. Begin scan position-   52, 54, 56, 58. Interim scan position-   60. End scan position-   64. Turntable-   66. Arm-   68. Rotating member-   70. Vertical support-   72. Start position-   74. Foot insert member-   80. Cover-   A. Central axis-   D1, D2. Distance-   L. Left knee-   Q. Axis-   R. Right knee-   R1, R2. Radius

What is claimed is:
 1. An apparatus for tomographic imaging, theapparatus comprising: a radiographic radiation source; a digitalradiographic detector; a turntable to move the source and detector abouta central axis; a detector support attached to the turntable and to thedetector to support the detector as the detector moves about the centralaxis, the turntable having at least a portion thereof spaced from thecentral axis at a same distance as the detector is spaced from thecentral axis; a source support attached to the turntable and to thesource to support the source as the source moves about the central axis,the source support having at least a portion thereof spaced at adistance from the central axis greater than the distance that theturntable is spaced from the central axis; a C-shaped housing configuredto enclose the source as the source moves about the central axis and toenclose the detector as the detector moves about the central axis, theC-shaped housing comprising a gap configured to allow a subject to bepositioned at the central axis by moving the subject through the housinggap; and a housing extension attached to the housing and configured tocontrollably extend from the housing across the housing gap to enclosethe detector as the detector moves across the housing gap and to preventthe subject from moving through the housing gap, wherein the detector isconfigured to capture radiographic images of the subject positioned atthe central axis, which subject is radiographically exposed using theradiographic radiation source.
 2. The apparatus of claim 1, wherein theturntable includes at least a portion thereof in the shape of a circulararc.
 3. The apparatus of claim 1, wherein a detector path distance asmeasured from the central axis to a point on a detector path isequivalent to a turntable distance as measured from the central axis toa point on the turntable.
 4. The apparatus of claim 1, wherein theturntable includes a portion thereof shaped in a circular arc having acenter at the central axis.
 5. The apparatus of claim 1, wherein thesource support comprises a support arm extending away from the centralaxis, and wherein the source is attached to a distal end of the supportarm furthest from the central axis.
 6. The apparatus of claim 5, whereinthe support arm comprises a straight, linear, rigid configuration. 7.The apparatus of claim 6, wherein the C-shaped housing is furtherconfigured to enclose the source support, the detector support and theturntable as the source moves about the central axis and as the detectormoves about the central axis.
 8. The apparatus of claim 7, wherein thegap in the C-shaped housing is further configured to allow the subjectto be positioned at the central axis by moving the subject through thehousing gap in a direction substantially perpendicular to the centralaxis.
 9. An apparatus for cone beam computed tomographic imaging of oneextremity of a patient, the apparatus comprising: a digital radiationdetector configured to move around a central axis along a detector pathat a first radial distance from the central axis; a radiation sourceconfigured to move around the central axis along a source pathsimultaneously with the movement of the detector along the detectorpath, the source path at a second radial distance from the central axis,the second radial distance greater than the first radial distance; aC-shaped housing configured to enclose the source as the source movesaround the source path and to enclose the detector as the detector movesaround a portion of the detector path, the housing comprising a gapconfigured to allow the one extremity of the patient to be positioned atthe central axis by moving the one extremity of the patient through thehousing gap in a direction substantially perpendicular to the centralaxis; and a housing extension attached to the housing and configured tocontrollably extend from the housing across the housing gap to enclosethe detector as the detector moves along the detector path and acrossthe housing gap and to prevent the one extremity of the patient frommoving through the housing gap.
 10. The apparatus of claim 9, furthercomprising a turntable to support both the detector and the source asthe detector and the source move around the central axis, the turntablehaving at least a portion thereof in a shape of a circular arc.
 11. Theapparatus of claim 10, wherein a distance as measured from the centralaxis to a point on the detector path is equivalent to a distance asmeasured from the central axis to a point on the turntable.
 12. Theapparatus of claim 9, wherein the detector is configured to captureradiographic images of a subject positioned at the central axis when thesubject is radiographically exposed using the radiation source.
 13. Theapparatus of claim 10, wherein the turntable portion in the shape of thecircular arc comprises a center coinciding with the central axis. 14.The apparatus of claim 10, further comprising a source support armattached to the turntable and to the source, wherein the source supportarm extends away from the central axis, and wherein the source isattached to a distal end of the support arm furthest from the centralaxis.
 15. The apparatus of claim 14, wherein the source support armcomprises a straight, linear, rigid configuration.